Once you understand how the changeover device works, the next question is usually how the system should actually be plumbed, particularly when councils like Auckland Council, under its SMAF rules, require both stormwater detention and reuse from the same tank.
The good news is that whether it is a SMAF dual-use setup, a simple rain-harvest tank or a domestic emergency reuse system, the plumbing logic is almost identical. The main differences are in how outlets are controlled and how compliance is documented. If you have not read the previous part of this series, it covers selecting the right pump and changeover device.
Typical Layout: How It Connects
Three runs make up the system, and it helps to read them as three separate problems rather than one diagram.
Roof to tank: roof, then leaf guards, then a first-flush diverter if one is fitted, then the tank inlet.
Tank to house: tank outlet, then pump, then changeover device, then the house supply line feeding toilets, laundry and outdoor taps.
Tank overflow: tank overflow to an approved stormwater outfall.
The first run decides what quality of water arrives. The second decides what happens when the tank runs dry. The third decides what the council sees, and it is the one most often left until last. Typical plumbing schematics are available through our specification tools, including the Slimline Specification Tool.
Key Details to Get Right
- Mount the pump at or near the tank outlet.
- The changeover device automatically switches between tank and mains.
- Connect the overflow to an approved stormwater disposal point, not to ground soakage unless consent allows.
- Label non-potable outlets "Rainwater, Not for Drinking" and use lilac (purple) pipework for reuse lines.
Each of these has a reason behind it. The overflow connection point is the difference between a system a council will sign off and one it will not, and it is decided by where the tank sits, so it is worth settling early. The labelling and lilac pipework are for whoever works on the house next, who will not have seen the plans and needs to know at a glance which line carries what. For additional regulatory guidance, refer to Auckland Council or your own local council.
Backflow Prevention: Non-Negotiable
Whenever mains water is connected to a non-potable supply, backflow prevention is required under New Zealand regulations. The concern is the direction of flow: without protection, water from the non-potable side can travel back towards the mains, and that is a problem well past the boundary of the property.
Promax changeover devices include a dual-check valve that provides suitable protection for low-risk domestic reuse supplying toilets, laundry and outdoor taps. Some installers choose an RPZ valve; however, this is typically unnecessary for low-risk domestic applications. What matters at consent stage is that your schematic clearly shows the dual-check configuration, because the reviewer is reading the drawing, not the fitting.
SMAF-Compliant Dual-Use Systems
In Auckland's SMAF zones, tanks must deliver both retention for reuse and detention to slow runoff leaving the property. One tank, two jobs, and the tank is divided by height to do them.
- The upper portion of the tank, the detention volume, drains slowly via a calibrated orifice.
- The lower portion, the retention volume, is pumped for reuse.
- The changeover device maintains supply to toilets and laundry when retention water runs out.
The two volumes work on opposite logic, which is the part that catches people out. Detention is useful empty, because it needs headroom before the next storm. Retention is useful full, because it is the water you are going to use. Splitting the tank by height is what lets one vessel do both. Our article on SMAF stormwater rules covers the requirement in more detail.
Documenting the System for Consent
A compliant system that is not documented is not a compliant system as far as the council is concerned. To satisfy consent, document:
- Detention and retention volume calculations
- Orifice size and invert level
- Backflow device type
- Lilac pipework and signage
- Manufacturer specifications for tank and pump
Assemble this at design stage rather than after installation. Every item on that list is a decision you make when you choose the tank, the pump and the layout, so writing it down as you go costs nothing, while reconstructing it afterwards means going back to things that are already plumbed in or buried. A SMAF calculator is available to assist with the design inputs.
Signage, Labelling and Maintenance
- Label non-potable outlets and use lilac pipework
- Keep leaf guards and overflow screens clear
- Check the changeover device annually for correct switching
- Clean filters and first-flush diverters, especially after long dry periods
The annual changeover check is the one worth putting in a calendar. A changeover device that has stopped switching correctly does not announce itself, because the house keeps getting water either way. The check is what tells you it is still doing its job. Long dry periods matter to the filters and the first-flush diverter for the same reason they matter at the roof: that is when the most material has had time to build up. The final part of this series covers maintenance, troubleshooting and handover in full.
Bringing It Together
With the right changeover device, lilac header pipework and documented schematics, you will achieve a council-compliant, sustainable and low-maintenance rainwater reuse system that switches automatically between tank and mains supply. The plumbing itself is not complicated, and it barely changes between a simple rain-harvest tank and a SMAF dual-use install. What changes is how much of it you have to prove on paper. Decide the layout, the backflow device and the documentation at the same time as you decide the tank, and the compliance side stops being a separate exercise at the end.